A letter from the Joint Council for Qualifications, which represents Britain's major exam boards, urged ministers 'in the strongest terms' to delay the diplomas.

They said introducing them too rapidly could destroy 'standards and quality' and undermine their value to universities and employers.

Headmasters and academics warn that the pace of change in the education system is risking complete meltdown.

Business leaders say they are yet to be convinced Labour's diplomas will add anything beyond what is already covered in traditional exams.

Diplomas are being introduced over the next two years to secondary schools across the country.

The qualifications - which will combine work-based training with classroom study - were pioneered last September in five practical areas, including health, media and engineering.

But they were taken by just 12,000 students - around a quarter of the original estimate.

Ten days ago, it also emerged one of the courses would be delayed by a year.

Ministers said science diplomas for sixth-formers would be introduced in September-2012 not 2011, saying they needed 'further work'.

The JCQ letter said: 'The original timescale could only be achieved if we now compromise the quality of development in the areas of assessment and standards. We urge you in the strongest terms to defer implementation.'

Despite this, the Department for Children, Schools and Families is pushing ahead with the latest phase of diplomas.